A method of the type defined above is disclosed in: R. A. Sones, G. T. Barnes, "A method to measure the MTF of digital X-ray systems"; Med. Phys. 11(2), March/April 1984.
This article describes how in a digital imaging system, for example a radiography or fluoroscopy system, the modulation transfer function, abbreviated to MTF hereinafter, is determined. The MTF of an imaging system is an objective measure of the imaging quality thereof. In the absence of geometrical distortion an imaging system will display a sinusoidal intensity distribution on a detector input plane as a sinusoidal intensity distribution whose contrast is reduced and whose phase is shifted relative to the original intensity distribution. By dividing the ratio in the displayed intensity distribution by the contrast of the intensity distribution on the input plane of the detector, the MTF of the detector can be measured for a plurality of spatial frequencies. For a spatial frequency zero the MTF is 1, and decreases versus an increasing frequency to 0.
A more efficient manner of measuring the MTF of an imaging system is based on the fact that the MTF can be written as the modulus of the one-dimensional Fourier transform of the line spread function of the imaging system. The line spread function describes the image of a line, displayed by the detector, on the detector input plane. With digital detectors whose detector input plane is subdivided into a matrix of individual detection sub-planes, or with imaging systems having an analog detector in which the detector signal values are stored in a digital memory, the problem arises that when the MTF is determined from the Fourier transform of the line spread function, aliasing occurs because of too low a sampling frequency. Aliasing is the occurrence of components of the spectrum of the displayed image at lower frequencies than the frequency to which the spectral components actually belong. The reason is that in the spatial frequency domain the spectrum of a sampled signal is a periodical version of the actual spectrum. When these periodic spectra overlap, reconstruction of the original spectrum is not possible. By structuring the test object as a plurality of equidistant parallel line elements in the form of radio-transparent narrow slits or radio-opaque thin wires having an element width substantially less than half the center-to-center spacing between line elements the center-to-center spacing with reference to the detector being equal to (n+1/2) times the inverse of the spatial sampling frequency, n being a natural number, the aliasing problem is obviated and the MTF can be determined by Fourier transofrming the detector signals which are proportional to the image to be displayed. A problem which occurs when, for example, an X-ray image intensifier tube is used as the detector, which is optically coupled to a television pick-up tube, is that scanning the input screen of the television pick-up tube may introduce an asymmetry in the line spread function.